[mm v2 0/3] Support memory cgroup hotplug

[Balbir Singh]

On 23/11/16 19:07, Michal Hocko wrote:
> On Wed 23-11-16 18:50:42, Balbir Singh wrote:
>>
>>
>> On 23/11/16 18:25, Michal Hocko wrote:
>>> On Wed 23-11-16 15:36:51, Balbir Singh wrote:
>>>> In the absence of hotplug we use extra memory proportional to
>>>> (possible_nodes - online_nodes) * number_of_cgroups. PPC64 has a patch
>>>> to disable large consumption with large number of cgroups. This patch
>>>> adds hotplug support to memory cgroups and reverts the commit that
>>>> limited possible nodes to online nodes.
>>>
>>> Balbir,
>>> I have asked this in the previous version but there still seems to be a
>>> lack of information of _why_ do we want this, _how_ much do we save on
>>> the memory overhead on most systems and _why_ the additional complexity
>>> is really worth it. Please make sure to add all this in the cover
>>> letter.
>>>
>>
>> The data is in the patch referred to in patch 3. The order of waste was
>> 200MB for 400 cgroup directories enough for us to restrict possible_map
>> to online_map. These patches allow us to have a larger possible map and
>> allow onlining nodes not in the online_map, which is currently a restriction
>> on ppc64.
> 
> How common is to have possible_map >> online_map? If this is ppc64 then
> what is the downside of keeping the current restriction instead?
> 

On my system CONFIG_NODE_SHIFT is 8, 256 nodes and possible_nodes are 2
The downside is the ability to hotplug and online an offline node.
Please see http://www.spinics.net/lists/linux-mm/msg116724.html

>> A typical system that I use has about 100-150 directories, depending on the
>> number of users/docker instances/configuration/virtual machines. These numbers
>> will only grow as we pack more of these instances on them.
>>
>> From a complexity view point, the patches are quite straight forward.
> 
> Well, I would like to hear more about that. {get,put}_online_memory
> at random places doesn't sound all that straightforward to me.
> 

I thought those places were not random :) I tried to think them out as
discussed with Vladimir. I don't claim the code is bug free, we can fix
any bugs as we test this more.

Balbir Singh.